# ---
# title: 77. Combinations
# id: problem77
# author: Tian Jun
# date: 2020-10-31
# difficulty: Medium
# categories: Backtracking
# link: <https://leetcode.com/problems/combinations/description/>
# hidden: true
# ---
# 
# Given two integers _n_ and _k_ , return all possible combinations of _k_
# numbers out of 1 ... _n_.
# 
# You may return the answer in **any order**.
# 
# 
# 
# **Example 1:**
# 
#     
#     
#     Input: n = 4, k = 2
#     Output:
#     [
#       [2,4],
#       [3,4],
#       [2,3],
#       [1,2],
#       [1,3],
#       [1,4],
#     ]
#     
# 
# **Example 2:**
# 
#     
#     
#     Input: n = 1, k = 1
#     Output: [[1]]
#     
# 
# 
# 
# **Constraints:**
# 
#   * `1 <= n <= 20`
#   * `1 <= k <= n`
# 
# 
## @lc code=start
using LeetCode

## add your code here:
## @lc code=end
